1. Field of the Invention
This invention relates to a novel process for the preparation of aqueous dispersions or solutions of oligourethanes or polyurethanes containing auxiliary agents and additives which are neither dispersible nor soluble in water, and the use of these solutions or dispersions as coating compounds for flexible or rigid substrates.
2. Description of the Prior Art
The preparation of stable, aqueous polyurethane-polyurea dispersions in known in the art and is described, for example, in German Offenlegungsschriften Nos. 1,495,745; 1,770,068; 2,555,534; 2,446,440 and 2,543,091; U.S. Pat. Nos. 3,479,310; 3,756,992 and 4,108,814 and D. Dieterich et al, Angew. Chem. 82, 53 (1970).
These processes of preparation may be subdivided into those which depend on the use of solvents and the so-called solvent-free processes.
In the processes which require the use of solvents, high molecular weight polyurethanes are synthesized by polyaddition in an organic solution, i.e., in a homogeneous phase, before they are dispersed in water. This method results in exceptionally high quality polyurethane films which satisfy even the stringent requirements for textile coating compounds.
Among the solvent-free processes, two should be particularly mentioned, namely, the so-called solvent-free dispersion process, which has been outlined in principle, e.g., in German Offenlegungsschrift No. 1,770,068 and U.S. Pat. No. 3,756,992 and the process described in German Offenlegungsschriften No. 2,446,440 and U.S. Pat. No. 4,108,814 in which a prepolymer containing sulphonate and isocyanate groups is subjected to a reaction with a diamine accompanied by chain lengthening during or after the dispersion process.
These aqueous dispersions have all of the advantages of polyurethanes, such as good processing characteristics and excellent physical fastness properties.
On the other hand, they often have disadvantages, such as poor, rubber-like handle when applied as top coats and insufficient levelling power of the dispersions.
There have therefore been many proposals to obviate these disadvantages by the addition of certain auxiliary agents and additives. Thus, for example, in German Offenlegungsschrift No. 2,138,016 and in U.S. Pat. Nos. 3,816,168 and 3,823,102, it is recommended to improve the handle of dissolved polyurethanes by the addition of nitrocellulose or other auxiliaries. These methods are substantially limited to the use of organic solutions of the auxiliary agents to modify dissolved polyurethanes. In German Offenlegungsschrift No. 2,542,767 it is recommended that nitrocellulose and/or cellulose acetobutyrate enriched with plasticizers and emulsifiers and dissolved in organic solvents should be added to an organic solution of a polyurethane and the resulting combination should then be converted into an aqueous dispersion. In this case the solvent remains in the aqueous dispersion, with the result that the well known disadvantages of systems containing solvents (fire characteristics, environmental problems) are not overcome.
German Offenlegungsschrift No. 1,770,068 describes the modification of polyurethane dispersions with various polymers, pigments and the like, in particular for modifying a polyurethane which has already been dispersed. In this method of modification, the dispersion contains two products side by side which often separate if the substance added is not dispersible in water. Furthermore, it is sometimes necessary to add plasticizers and emulsifiers, e.g., when nitrocellulose is used.
In the case of aqueous dispersions which have been prepared using acetone, for example, by the solvent process according to German Offenlegungsschrift No. 1,495,745 or No. 1,694,175, substances which are insoluble in water but soluble in acetone, for example, may be incorporated. The acetonic solution is converted into a homogeneous dispersion with water, and the solvent is subsequently evaporated off.
The problem, however, remained that additives which were insoluble in water and incapable of being dispersed in water without the addition of emulsifiers were required to be incorporated in aqueous polyurethane dispersions without the use of readily inflammable solvents so that such dispersions suitable for storage could be obtained. One particular problem was that auxiliary agents for the dressing of leather, in particular, those of the type exemplified under 1 and 2, below, which are neither soluble nor dispersible in water and often insoluble or only difficultly soluble in organic media were required to be incorporated in solvent-free, aqueous polyurethane dispersions suitable for the dressing of leather, in particular as leather finishes, to produce storage stable dispersions containing the aforesaid auxiliary agents.
These problems underlying the present invention could surprisingly be solved by the process according to the invention which is described below.